The present invention relates to a discharging member and a charging device using the same which are applicable to an image forming apparatus of the type having a photoconductive element or similar image carrier or an electrostatic recording member or similar electrostatic recording member.
An electrophotographic copier, facsimile machine, laser printer or similar electrophotographic recording apparatus has a photoconductive element or similar image carrier or an electrophotographic recording sheet or similar recording medium, and a charging device for charging the image carrier or the recording medium. Typical of charging devices known in the art are a corotron charger and a scorotron charger which belong to a family of corona chargers. Such a corona charger has a casing, a charging member in the form of a thin wire which is made of tungsten or molybdenum and arranged at the center of the casing, and a high-tension power source for applying a high voltage across the wire and casing. The wire is located at a predetermined distance from the photoconductive element or similar object to be charged. When a high voltage is applied across the wire and casing, a corona discharge occurs between the wire and the casing. Then, ions are emitted from the wire and deposited on the object to thereby charge the latter. A problem with this kind of corona charger is that about 80% of the released ions flows toward the casing, resulting in poor charging efficiency. Should the casing be absent, however, it would be difficult to effect the discharge unless an extremely high voltage was applied across the wire and casing. Reducing the distance between the wire and the object may be successful in lowering the required voltage. This, however, brings about another problem that unnoticeable undulations on the surface of the object are apt to cause a non-uniform charge distribution or undesired spark discharge to occur.
Discharges in the atmosphere are always accompanied by the generation of ozone. A large amount of ozone is injurious not only to health but also to mental hygiene because it has an offensive smell. Even the discharge between the wire and the casing as mentioned above produces ozone. Therefore, enhancing efficient use of ions issuing from the wire is desirable in reducing harmful ozone. Besides, the discharge start voltage increases with the increase in the distance between the wire and the object and with the decrease in the distortion of an electric field which is developed near the wire. Then, a higher voltage would be required to insure a necessary amount of charge and a necessary discharge current, aggravating the generation of ozone. From the fact that the discharge start voltage increased with the decrease in the distortion of the electric field which is developed near the discharging member, i.e., wire, it will be seen that a conductive and/or flat discharging member increases the amount of ozone. A drawback particular to a corotron charger or similar charger using a thin wire is that the wire itself is not mechanically strong and is apt to loose elasticity due to aging such as oxidation and degeneration. In the worst case, such a wire will be broken while the apparatus is in operation.